1. Field of the Invention
The present invention relates to a magnetic tape system. More particularly, the present invention relates to a magnetic tape system including a buffer memory in which a plurality of commands and data can be stored and prefetch control means in which data from a host system can be prefetched into the buffer memory.
2. Description of the Related Art
A magnetic tape system is extensively used as an external memory unit of a computer and, recently, the magnetic tape system is especially used as a backup system for an external memory unit such as a magnetic disk unit. At present, a prefetch control unit is disposed in this magnetic tape system. The prefetch control unit includes a command buffer memory for temporarily storing a plurality of commands from a host system and a data buffer memory for temporarily storing data from the host system to be stored or data read out. Accordingly, if commands of the host system are received in the memory of the prefetch control unit, the host system can conduct subsequent processing or can continue to generate magnetic tape access commands before termination of access in the drive unit, while the drive unit can perform processing before receipt of commands from the host system or confirmation thereof. Therefore, by introduction of the prefetch control unit, the entire processing capacity of the magnetic tape system can be improved.
Since the capacity of the command buffer memory or data buffer memory is limited, if many commands from the host system must be received, prefetching must be restricted.
In the conventional prefetch control, the restriction of prefetching is defined by consecutive steps such as 8 KB, 16 KB, 32 KB, and 64 KB. If the value of the restriction is 8 KB, the command buffer memory is occupied by 8 KB for a prefetch request smaller than 8 KB. When the value of the restriction is 32 KB, the prefetch request for 8 KB may be accepted, however, it is deemed that 32 KB of the command buffer memory is used for the prefetch request for 8 KB, resulting in a low availability of the command buffer memory. Generally, a small amount of restriction may provide more opportunities of acceptance of the prefetch. But, a small restriction must be changed to a large restriction when the prefetch request for a large amount of data is required.
In the conventional prefetch control, however, the restriction is effected by exchanging values predetermined stepwise in relation to one another, and every time exchange is performed, an error report is transmitted to the host system. Accordingly, error reports are often transmitted, and the host system is withheld for a certain time from transmitting data and commands to the prefetch control unit. For this reason, the conventional prefetch control is defective in that the inherent function of the prefetch control unit is not sufficiently utilized. Moreover, although prefetching must be stopped at the end of the magnetic tape, a satisfactory measure for effecting this stop has not been developed.
These problems will be described hereinafter with reference to the accompanying drawings.